An organic light-emitting diode device, also called an OLED, commonly includes an anode, a cathode, and an organic electroluminescent unit sandwiched between the anode and the cathode, all of which is disposed on a substrate. The substrate is often made from a transparent material such as plastic or glass. The organic electroluminescent unit generally includes a hole-transporting layer, a light-emitting layer, and an electron-transporting layer, among others. A power source is attached to the anode and cathode. OLEDs are attractive as a light source in lighting applications because of e.g., their low drive voltage, high efficiency, high luminance, long life, thin profile, large-area diffuse light emission, and ability to be shaped into a curved configuration if disposed on a flexible substrate.
When a proper voltage is applied, the anode injects holes and the cathode injects electrons into the organic electroluminescent unit. The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton” is produced, which is a localized electron-hole pair having an excited energy state. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.
OLEDs can emit different colors, such as red, green, blue, or white, depending on the emitting property of its light emitting layer. OLEDs emitting white light often contain individual colored emitters, such as a red-emitting dye, a blue emitting dye, and a green emitting dye, such that when the emission from the individual dyes is combined, the resultant light emission is white light. Efficient white light producing OLED devices are considered a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, office lighting, and the like. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33), although this can vary significantly depending on the desired color temperature for the application. Light sources that emit radiation with a correlated color temperature (CCT) in the range of 2500 K to 7000 K are generally considered to be white light sources, although this range is not deemed to be limiting.
Certain challenges exist, however, with the manufacture and use of OLEDs. It is very challenging to manufacture OLEDs of various CCT that have the desired performance characteristics described above, e.g. high brightness, high efficiency, long life, etc. There have been demonstrations of individually colored OLEDs of red, green, and blue having extremely long life and high efficiency but when trying to combine them into a single white light source the performance characteristics often drop significantly. It is advantageous to optimize an OLED stack, e.g. anode, cathode, transport layer(s) and emission layer(s) based on a specific emitter of a specific wavelength, as the performance will often be increased if the microcavity is tuned for that specific dye. Design considerations such as material composition and thickness of the various layers must be accounted for. Additionally, the materials chosen in the stack should be compatible from both a chemistry and energy level/transfer perspective. As such, when trying to make an OLED with multiple emitters to generate a white light source, it is difficult to optimize and achieve the same performance characteristics of individually colored OLEDs that could have their emitted light mixed to achieve the same color.
Due to the complex energy transfer mechanisms involved in white OLEDs that contain multiple dyes, one cannot simply change the relative concentrations of the dyes to achieve a different CCT of white light. Therefore, from a design and manufacturing standpoint, each white OLED of different CCT would need to be optimized in the lab from a performance standpoint and then optimized from a manufacturing standpoint. If color mixing in a luminaire is utilized as described herein, only the individually colored OLEDs, such as red, blue, and green would need to be optimized and manufactured. The relative intensities of each component would then be selected based on relative intensity, power, and distance, to generate white light that is capable of having a broad range of CCTs. As an alternative, a white OLED could be tuned via color mixing with one or more individually colored OLEDs to achieve the particular CCT desired for the application.
Lastly, due to the fact the OLEDs are a more recent lighting technology, material sets that allow for all of the color space at high performance does not exist. For example, deep blue dyes are not as mature from a performance standpoint when compared to green dyes that are used in OLEDs.
Accordingly, a method of increasing the color space available when illuminating with one or more OLEDs would be useful.